Method for forming grain boundary junction devices using high Tc superconductors

ABSTRACT

A method is disclosed for fabricating grain boundary junction devices, which comprises preparing a crystalline substrate containing at least one grain boundary therein, epitaxially depositing a high Tc superconducting layer on the substrate, patterning the superconducting layer to leave at least two superconducting regions on either side of the grain boundary and making electrical contacts to the superconducting regions so that bias currents can be produced across the grain boundary.

This application is a division of application Ser. No. 07/155,946 filedFeb. 16, 1988, now U.S. Pat. No. 5,162,298.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to devices employing high T_(c) superconductingmaterials, and more particularly to simple, practical devices employingthese materials, and to methods for making these devices. The devicesare planar structures employing selected grain boundaries in the highT_(c) superconducting materials as weak link or junction barriers. Suchstructures are reproducibly made with good operating margins.

2. Description of the Prior Art

Recently, the remarkable discovery by J. G. Bednorz and K. A. Mueller,reported in Z. Phys. B.--Condensed Matter 64, 189 (1986) and EurophysicsLetters, 3, 379 (1987) completely changed the direction and importanceof superconducting technology. Their discovery was that certain metallicoxides can exhibit superconducting transition temperatures considerablyin excess of 23 K. These materials are often termed "High T_(c)Superconductors". Since the initial discoveries of Bednorz and Mueller,a vast amount of research and development has been undertaken around theworld to further study these types of superconducting materials in orderto extend even farther the temperature range over which the materialsare superconducting, as well as to understand the basic mechanisms forsuperconductivity in this class of materials.

Bednorz and Mueller first showed superconducting behavior in mixedcopper-oxides, typically including rare earth and/or rare earth-likeelements and alkaline earth elements, for example La, Ba, Sr, . . . ,and having a perovskite-like structure. Materials including the socalled "1-2-3" phase in the Y-Ba-Cu-O system have been found to exhibita superconducting transition temperature in excess of 77K. R. B.Laibowitz and co-workers were the first to achieve and describe a methodfor making thin films of these materials. These thin film structures andmethods for making them are described in co-pending application Ser. No.027,584, filed Mar. 18, 1987, now abandoned and assigned to the presentassignee. The work of Laibowitz et al is also described in Phys. Rev. B,35, 8821 (1987). In this technique, a vapor transport process is used inwhich the components of the superconducting film are vaporized anddeposited on a substrate in an oxygen atmosphere, after which thedeposited film is further annealed.

Another paper describing thin films of these high T_(c) superconductors,and specifically high critical currents in these materials, is P.Chaudhari et al, Phys. Rev. Lett. 58, 2864, June 1987. Chaudhari et aldescribed epitaxial high T_(c) superconducting films formed onsubstrates such as SrTiO₃, in which the critical current at 77 K was inexcess 10⁵ A/cm².

Other references generally describing the deposition of films or layersof high T_(c) superconducting materials include J. Cuomo, co-pendingapplication Ser. No. 043,523, filed Apr. 28, 1987, now abandoned and A.Gupta, co-pending application Ser. No. 121,982, filed Nov. 18, 1987, nowU.S. Pat. No. 4,997,809 assigned to the present assignee. The first ofthese co-pending applications describes a plasma spray coating processwhile the second describes a method for coating a substrate, as byspraying from solution, and then patterning the coated film toeventually produce a patterned layer of high T_(c) superconductingmaterial.

Epitaxy of high T_(c) superconducting films has been accomplished onseveral substrates, including SrTiO₃. In particular, superconductingfilms capable of carrying high critical currents have been epitaxiallydeposited as noted in a paper by P. Chaudhari et al, published inPhysical Review Letters, 58, 2684, June 1987.

The initial work of Bednorz and Mueller has been extended to includeother copper oxide compositions which exhibit high temperaturesuperconductivity. These other compositions typically do not include arare earth element, but instead include an element such as Bi. Arepresentative material is one in the system Bi-Sr-Ca-Cu-O whichexhibits a drop in electrical resistance at about 115K and a transitionto zero resistance at 80K. Recently, C. Michel and co-workers reportedsuperconductivity in the non-rare earth containing BiSrCuO system withtransition temperatures as high as 22K. C. Michel et al, Z. Phys.B-Condensed Matter, 68, 412 (1987). A new BiSrCaCuO_(x) composition wasthen found by Maeda and Tanaker to exhibit high transition temperatureswith a resistivity completion in the 80K range and a well definedresistivity decrease at about 115K. This work was reported by theseauthors in a preprint, which is to be published in the Japanese Journalof Physics.

The work of Maeda and Tanaker was confirmed by C. W. Chu and co-workers,and by Hazen and co-workers, these researchers noting the structure andphase identification of this bismuth-including copper oxide system.Reference is made to C. W. Chu et al, Phys. Rev. Lett., Vol. 60, pages1174-1177 (Mar. 21, 1988).

The copper oxide superconducting materials exhibiting transitiontemperatures in excess of about 30° K are generally known as "high T_(c)superconductors", and will be referred to in that manner throughout thespecification. This designation is meant to include both the materialshaving rare earth or rare earth-like elements in their crystallinestructure, as well as the more recently reported materials which do notcontain rare earth or rare earth-like elements. Generally, all thesematerials are copper oxide based superconductors having Cu-O planes thatappear to be primarily responsible for carrying the supercurrents, wherethe copper oxide planes are separate or in groups separated by the otherelements in the compositions.

The advent of high temperature superconductivity should lead to numerousapplications of junction devices operating at temperatures much highthan those that have been achieved with superconducting devicesfabricated from conventional superconductors. However, fabrication ofworkable devices has not been easy. The first such report of an operabledevice, in this situation a SQUID device described by R. Koch et al.,utilized a film of high T_(c) superconductor in which high energy beamswere used to produce two localized constrictions to form weak linkconnectors between high T_(c) superconductors. In this manner, asuperconducting loop having weak link regions was created and operatedsuccessfully as a SQUID. This first high T_(c) superconducting deviceand the method for making it are described in a copending patentapplication to G. J. Clark et al., Ser. No. 7-037,912, filed Apr. 13,1987, now U.S. Pat. No. 5,026,682 and assigned to the present assignee.

Although it has been experimentally established that high T_(c)superconducting copper oxides, such as YBa₂ Cu₃ O_(7-x) can bereproducibly prepared as thin films, a well defined, all high T_(c)single junction exhibiting Josephson tunneling currents has not beensuccessfully fabricated. In such a device, two superconducting layerscomprised of high T_(c) superconductors are separated by a thin (10-50angstrom) layer which operates as a tunnel barrier. An oxide materialcan be used for the barrier layer. However, the high T_(c) copper oxidesuperconductors, whether fabricated as films or bulk samples, requireannealing in an oxygen atmosphere at high temperature, typically about900° C. This high temperature processing makes it extremely difficult ifnot impossible to deposit a counter electrode comprised of high T_(c)superconducting material over the very thin insulating tunnel barrier.Generally, the high temperature processing severely degrades thejunction quality. Such processing is also incompatible with most of theconventional lithographic patterning processes.

Another feature of these high T_(c) superconducting materials is theirextremely short coherence length, which is a measure of the distanceover which the superconducting pairing extends. In these high T_(c)superconductors the coherence lengths are typically 10-30 angstroms, incontrast with coherence lengths of 1000 angstroms in conventional priorart superconducting materials. Such low coherence lengths representanother technical obstacle to making either planar function or weak linktype tunnel barriers in, for example, micro-bridge Josephson junctiondevices. In weak link devices, a very narrow constriction operates as aweak link barrier between two large superconducting regions to provideJosephson-like characteristics. However, because the coherence length isso small in high T_(c) superconductors, the geometrical constrictionmust have a dimension of the order of the coherence length in order toexhibit weak-link characteristics. Such narrow constrictions cannot bereliably produced. When planar junctions are formed, it is also verydifficult to reliably deposit tunnel barrier layers having thicknessesof the order of the coherence length (about 10 angstroms) of high T_(c)superconductors.

Accordingly, it is a primary object of the present invention to providea practical device employing high T_(c) superconducting materials wherethe aforementioned problems are avoided.

It is another object of the present invention to provide a method forreproducibly making practical junction and weak link superconductingdevices employing only high T_(c) superconducting materials.

It is another object of this invention to provide a device and methodfor making the device employing high T_(c) superconducting materials ina planar configuration wherein the weak link or junction region can beprecisely located with a defined orientation.

In the practice of this invention, junction devices or weak link devicesare fabricated using a grain boundary between two high T_(c)superconducting grains. These grain boundaries are very narrow (aboutthe order of the unit cell in the materials, i.e., about 10 angstroms)and their electrical properties (such as resistance) can be readilyvaried to provide different device properties. In particular, a planarstructure is provided utilizing an epitaxial film of high T_(c)superconducting material deposited on a substrate having defined andpredetermined grain boundaries therein. In this manner, the grainboundaries in the substrate are reproducibly formed in the epitaxialsuperconductor film. Stated another way, epitaxy maps thepolycrystalline structure of the substrate into the high T_(c)superconductor film.

It is recognized that grain boundaries have been used to providepotential barriers for the flow of electrons thereacross in priorsuperconducting devices. Such devices have been called boundary layerJosephson junctions and have been described in the following references:

M. Ito et al, Japanese Journal of Appl. Physics, 21, No. 6, pp.L375-376, June 1982.

M. Ito et al, Appl. Phys. Lett., 43, (3) p. 314, August 1983.

T. Inamura et al, Jap. Journal of Appl. Phys., 21, Supplement 21-1, pp.313-318, 1982.

The devices described in these references use the grain boundaries thatrandomly occur when a superconductor film is deposited on a substrate.These superconductors are generally designated BPB films because theyare comprised of Ba, Pb, and Bi oxide combinations having aperovskite-type structure. These references do not teach a way tocontrollably make grain boundary junction devices whose characteristicscan be well controlled and which can be reproducibly formed with uniformproperties. As noted, these references describe devices in which arandom formation of randomly oriented grains occurs in materials havinglow transition temperatures of about 13K.

In further contrast with these and other references, the devices of thepresent invention are made in an epitaxial layer of high T_(c)superconducting material. Generally, epitaxy is thought of with respectto single crystal material rather than polycrystalline materials of thetype used for the substrate and the superconducting film in the devicesof this invention.

M. Suzuki et al describes the formation of planar Josephson-type devicesusing crystalline layers of BPB in J. Appl. Phys. 53, (3), p. 1622,March 1982. In this structure, two superconducting layers of BPB areseparated by an insulating tunnel barrier comprised of an insulatingoxide having the same crystal structure as BPB. Such device structureshave not been possible using high T_(c) superconducting materials, forthe reasons described above with respect to the high temperatureprocessing and very short coherence length in these new superconductors.

Accordingly, it is another object of the present invention to providepractical devices utilizing selected grain boundaries in high T_(c)superconducting materials.

It is another object of this invention to provide processing techniquesfor reproducibly making grain boundary superconductive devices employinghigh T_(c) superconducting materials, wherein the device properties areuniform and wherein the device structures are planar and easily andreproducibly fabricated.

It is another object of this invention to provide improved devicesemploying high T_(c) superconductors, wherein the design of such devicesand the techniques for making them effectively utilize features found innature which may otherwise be considered obstacles.

BRIEF SUMMARY OF THE INVENTION

This invention relates to improved devices utilizing high T_(c)superconducting materials and uniform, reproducibly created grainboundaries in such films for the fabrication of the devices. It isrecognized that grain boundaries have been utilized in the prior art astunnel barriers in the work relating to BPB oxides, and that thepossible presence of grain boundaries leading to Josephson tunnelingcurrents was mentioned in the copending application to G. J. Clark etal, Ser. No. 037,912. However, the present invention seeks to providedevices and methods for producing these devices which are controllableand reproducible to define grain boundary devices in high T_(c)Josephson materials having small coherence lengths. Further, thelocation, orientation, and number of these new grain boundary devicescan be predetermined and the properties of each of the devices can beadjusted during fabrication.

In a preferred embodiment, a substrate is prepared having at least onegrain boundary therein, which grain boundary is to be reproduced in anoverlying layer of high T_(c) superconductor. The layer of high T_(c)superconductor is then epitaxially deposited on the substrate in orderto reproduce in the superconducting layer the grain boundary present inthe substrate. This defines the location and orientation of the grainboundary in a controlled manner. After this, the superconducting film ispatterned to leave at least one superconducting region on each side ofthe grain boundary, these superconducting regions being used aselectrodes for current flow across the grain boundary. High energy beamsor excimer laser ablation can be used to define the superconductingregions that are to function as the electrodes for these superconductingdevices. After this, the superconducting regions are electricallycontacted and appropriately biased to have current flow across the grainboundary which functions as a potential barrier. A plurality of devicesof this type can be arranged in any manner to produce an array of suchdevices, a SQUID, etc.

These and other objects, features, and advantages will be apparent fromthe following more particular description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2, 3 and 4 schematically illustrate a process wherein a grainboundary device is fabricated in an epitaxial layer of high T_(c)superconducting material.

FIG. 5 schematically illustrates electrical connections to the deviceproduced by the technique illustrated in FIGS. 1-4.

FIG. 6 is a schematic illustration of a layer of high T_(c)superconducting material having two grains A and B separated by a grainboundary. Regions I, II, and III are defined by the dashed lines.

FIG. 7 is an illustration of the layer of high T_(c) superconductingmaterial shown in FIG. 6, where different regions I, II and III havebeen processed in grains A and B to illustrate the invention.

FIGS. 8, and 9A, 9B and 9C illustrate electrical properties of theregions I, II, and III shown in FIG. 7. More particularly, FIG. 8 is aplot of critical current I_(c) versus temperature for the three regionsI, II, and III, while FIGS. 9A-9C illustrate plots of critical currentI_(c) versus applied magnetic field H for the three regions I, II, III,respectively.

FIGS. 10A and 10B illustrate a process for fabricating a SQUID comprisedof 2 grain boundary devices in a superconducting loop of high T_(c)superconducting material.

FIG. 11 is a plot of critical current I_(c) versus applied magneticfield H for the SQUID illustrated in FIG. 10B, for three different biascurrents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the practice of this invention, superconducting devices comprised ofa single layer of high T_(c) superconducting material can be made in aplanar geometry utilizing grain boundaries for tunnel barriers or weaklink connections between superconducting grains. In contrast with theprior art, this can be done reproducibly and controllably since thegrain boundaries can be produced in the superconducting layers in amanner in which the orientation and location of the grain boundary arepredetermined.

The general process steps include the preparation of the substratehaving at least one grain boundary defined therein with respect to theorientation and location of the grain boundary. After this, a high T_(c)superconducting layer is epitaxially deposited on the substrate (or on athin interface layer epitaxially grown on the substrate) in order toproduce in the superconducting layer a grain boundary corresponding inlocation and orientation to the grain boundary in the underlyingsubstrate. After this, the superconducting layer is patterned to leaveregions of superconducting material on either side of the grain boundaryin order to produce a device having superconducting regions (electrodes)separated by the grain boundary. Electrical contacts can then be made tothe superconducting regions for connection to appropriate biasingsources. As will be seen, the properties of the grain boundary can beadjusted and multiple devices can be fabricated along a single grainboundary or along several grain boundaries.

Referring more particularly now to FIGS. 1-4, the general technique forproducing a planar grain boundary device is illustrated. In thistechnique, there is no subsequent processing step which would intereferewith the material properties of any of the component parts of thedevice, and the structure that is obtained is planar. The grain boundaryfunctions as a Josephson tunneling barrier or weak link connection, andis typically about 10 angstroms in width in these high T_(c)superconducting materials. More generally, the grain boundary width isof the order of the unit cell of these high T_(c) superconductors. InFIG. 1, substrate 10 includes two single crystal grains A and Bseparated by a grain boundary GB. This grain boundary is approximately10 angstroms in width and is schematically illustrated by the stipledregion between the crystalline grains A and B.

In FIG. 2, a layer 12 high T_(c) superconducting material has beenepitaxially deposited on the substrate 10 using, for example, knowntechniques. These techniques include vapor deposition as by evaporationor sputtering from multisources as described in the aforementionedarticles and copending patent applications in the names of R. B.Laibowitz, P. Chaudhari and others. Because the superconducting layer 12is epitaxially formed on the substrate 10, it will have crystallineregions A and B coextensive with those in the substrate 10, and a grainboundary GB having the same orientation and location as the grainboundary in the underlying substrate 10.

In FIG. 3, the superconducting layer 12 is patterned, for example byusing photons or high energy particles, represented by the arrows 14.This patterning can be done in a variety of ways, and is used to defineregions of the superconducting layer 12 which are to be left in theirsuperconducting state while the irradiated portions are eitherphysically removed or converted to a nonsuperconducting (i.e., normal)state. In order to render an irradiated region nonsuperconducting, thetechnique described by G. J. Clark et al in aforementioned copendingapplication Ser. No. 037,912 can be used. This technique is more fullydescribed in two publications by G. J. Clark et al, appearing in Appl.Phys. Lett. 51, (1987) at pages 139 and 1462, and comprises the use ofhigh energy beams to cause damage in high T_(c) superconductingmaterials. This damage can change the properties of the material fromsuperconducting to normal and even to a nonsuperconducting insulatingstate having an amorphous structure. Examples of high energy beams thatcan be used for this purpose are directed ion beams comprising ions suchas oxygen, As, Kr, etc. The ion beam can be directed to selected areasof the superconducting layer through the use of a mask. An example of aSQUID device made by this technique is described in R. H. Koch et al,Appl. Phys. Lett., 51, p. 200 (1987).

High energy beams providing energies in the range of about 250 KeV--2 or3 meV will typically provide enough damage to alter the properties ofhigh T_(c) copper oxide superconductors. However, it is within thepractice of this invention that patterning can be accomplished by energybeam irradiation where the superconducting material is not totallyconverted to a nonsuperconducting state, but rather has its criticaltransition temperature T_(c) lowered appreciably with respect to thenonirradiated regions of the superconducting layer.

Another technique for patterning high T_(c) superconducting layers isthe use of excimer ablative photodecomposition in which ultravioletradiation impinges on the superconducting layer to ablate (i.e., blowaway) the irradiated regions. Ablative photodecomposition will occur ifthe energy fluence of the UV radiation is sufficiently high that thethreshold for ablative photodecomposition is exceeded. In this process,the ablation occurs with substantially no thermal effect to thesurrounding nonirradiated regions. This is a particularly goodtechnique, as the surrounding regions will have the same superconductingproperties after patterning has occured.

FIG. 4 illustrates the structure that remains after patterning. Here, athin strip of the superconductor 12 is left on the substrate 10, thesuperconductor 12 including grains A and B separated by the grainboundary GB in the superconducting material. As is apparent, it is aplanar structure wherein grains A and B can be used for electricalcontacts, the current flow being substantially normal to the plane ofthe grain boundary.

Thus, the general steps of this process include the provision of asubstrate having at least one grain boundary therein whose location andcrystal orientation are predetermined, the epitaxial deposition of alayer of high T_(c) superconducting film on the substrate to establishin the superconducting film a grain boundary corresponding to that inthe substrate, patterning of the superconductor to leave superconductingregions separated by a portion of the grain boundary, and contacting thesuperconducting regions with the appropriate electrical sources. Oneexample of the final structure is shown in FIG. 5.

In FIG. 5, electrical contacts 16 are made to superconducting grains Aand B and a bias source, represented by battery 18 is attached theretofor providing a current flow across the grain boundary GB in thesuperconducting layer.

The substrate materials are selected from those materials on which anepitaxial layer of high T_(c) superconducting film can be deposited.Examples of suitable substrates include SrTiO₃ substrates for epitaxialdeposition thereon of high T_(c) superconducting materials such as YBa₂Cu₃ O_(7-x). Other suitable substrates will be apparent to those ofskill in the art, the substrates being generally chosen to havesufficient lattice match with the desired high T_(c) superconductingmaterial that the superconducting material can be epitaxially depositedthereon.

Techniques exist in the art for providing substrates having controlledgrain boundary growth. For example, a grain boundary with a controlledmisorientation can be obtained by forming a bicrystal from two orientedsingle crystals. The bicrystal is grown by sintering two single crystalpieces in a powder compact. During sintering, the single crystal piecesgrow at the expense of the smaller surrounding grains until the singlecrystals impinge on each other to form a single grain boundary. Thistechnique has been used to form the bicrystal of SrTiO₃ at a sinteringtemperature of 1600° C. Alternatively, a bicrystal can be formed by hotpressing two single crystal pieces together. While both techniques canbe used to form a long, well-defined grain boundary, the advantage ofthe second method is the ability to form a straight grain boundary thatis free of pores.

As was described, the grain boundary forms a tunneling barrier or weaklink between the superconducting grains to which electrical contact ismade. The critical current density and tunneling characteristics of thegrain boundary can also be modified with a low temperature (less than400° C.) annealing step in a controlled gas atmosphere. Both inert andreducing gasses, such as He and Ar, as well as reactive gasses such asCO₂ are effective for this purpose. An inert gas annealing step acts toreduce the oxygen content of the superconductor film. A CO₂ anneal willpromote the formation of BaCO₃ in a film of YBa₂ Cu₃ O_(7-x), whereBaCO₃ is an insulator. Since the activation energies for diffusion andsolid state reaction of grain boundaries are typically lower than forthe rest of the lattice, an optimum set of annealing temperatures andtimes exist for which the transition temperature T_(c) and normal stateresistivity of the grain boundary are altered while leaving thecorresponding properties of the adjacent superconducting grainsrelatively unaffected.

EXAMPLES (FIG. 6-9C)

The principles of this invention were demonstrated by the fabrication ofseveral Josephson junctions using a grain boundary in the high T_(c)superconductor YBa₂ Cu₃ O_(7-x). For this, a polycrystalline layer ofhigh T_(c) superconductor having a large grain size in the plane of thefilm was fabricated. This superconducting material was epitaxially grownon a substrate of SrTiO₃. Several substrates of polycrystalline SrTiO₃,having grains as large as 200-300 microns, were used on which the YBaCuOsuperconducting films were deposited.

These large grain SrTiO₃ substrates were prepared by sinteringcold-pressed pellets of fine-grained powder (average grain sizeapproximately 2.5 microns) in air at temperatures in the range1600°-1650° C., for at least 48 hours. These sintering conditions causeexagerated grain growth to occur, which leads to the formation of verylarge grains in the dense pellets (p/p_(th) ≧99%). The strontiumtitanate powder was prepared by reacting high purity powders ofstrontium carbonate and titanium dioxide at 1450° C. until a singlephase material is obtained.

To fabricate a single Josephson junction containing a well defined grainboundary as the weak-link bridge, high T_(c) YBa₂ Cu₃ O_(7-x) films wereepitaxially deposited onto the large grain polycrystalline SrTiO₃substrates. The details of superconductor film deposition and postdeposition annealing are those which have been described previously byR. B. Laibowitz, P. Chaudhari, et al. After this annealing step thesuperconducting films were epitaxially aligned with the grainorientation of the substrate, resulting in a large-grainedsuperconducting film. This is illustrated in FIG. 6 where thesuperconducting film 20 is epitaxially aligned with the substrate 22.Large grains A and B are produced in the superconducting film 20, wherethe grains are separated by the grain boundary GB.

In order to define the geometry and dimensions of a grain boundaryjunction device and its electrode pads, the technique of laser ablationwas used. Grains A and B were patterned by excimer laser ablation asdescribed hereinabove in order to make 3 lines I, II, and III, asillustrated in FIG. 7. In these examples, the substrate-superconductingfilm combination was mounted into a computer controlled stepping stageand irradiated with a demagnified image of variable size rectangularaperture. This technique can be used to pattern high T_(c)superconducting films in dimensions ranging from several centimeters inlength to submicron dimensions in width without any degradation incritical temperature T_(c) and critical current density J_(c).

In these examples, three narrow lines I, II, III having dimensions ofabout 20 microns×80 microns×0.5 microns thickness were patterned insuperconducting film 20. This step removed the superconducting materialsin the irradiated regions. The structure produced is shown in FIG. 7where line I is totally in grain A, while line III is totally in grainB. However, line II straddles the grain boundary GB. In laboratorydemonstrations, the width of the lines was varied from approximately 1micron to approximately 2 microns. The length of the line within a grainwas approximately 40 microns while the length of the line crossing thegrain boundary was varied between 2 and 40 microns.

The electrical characteristics of the 3 lines I, II, and III are shownin FIGS. 8 and 9A-9C. In FIG. 8 the critical current I_(c) is plottedagainst temperature for current flow in each of the three lines I, IIand III. From this FIG. it is apparent that line II, containing a grainboundary, always has a lower critical current than that lines I and III.

FIGS. 9A-9C more dramatically illustrate the essential features of aJosephson weak link junction in line II, in contrast with lines I andIII. These FIGS. plot critical current I_(c) versus applied magneticfield H for each of the three lines I, II and III. The plots in FIGS. 9Aand 9C are similar, while the plot in FIG. 9B clearly illustrates thepresence of the grain boundary junction in line II. The junctionresistance here is of the order of a few ohms and its capacitance isestimated to be a fraction of 1 picofarad. The Stewart McCumberparameter is of the order of 1 for these samples. Therefore, thehysterises in the I-V curves for these samples is quite small, in sharpcontrast to the conventional overlap junctions with higher capacitancemade by separating two superconducting layers with an insulating layerof, for example, an oxide material.

In the practice of the present invention, the number of grainboundaries, their orientations and their locations in thesuperconducting layer are known in advance of patterning for deviceformation. This is a major distinction over the randomness with respectto location and orientation of prior art grain boundary devices usingother types of superconductors. Another major distinction is withrespect to the superconducting grain size that is used in the presentinvention in contrast with that of prior art grain boundary devices. Inthe present invention, the grain sizes are typically hundreds of micronsin contrast with an average of about 10 microns grain size for prior artdevices. Because the grains in the present invention are very large, itis easy to isolate grain boundaries for patterning and formation ofdevices. This cannot be achieved in the prior art where the grainboundaries are extremely short and randomly oriented.

Another advantage of the present devices in contrast with prior artgrain boundary devices relates to the unformity of properties along thegrain boundaries. In the present invention, long grain boundaries can beproduced so that the same grain boundary can be used when patterningseveral devices, or an array of devices. Since the use of the same grainboundary as a barrier in more than one device helps to ensure theuniformity of individual device properties, the quality of circuits andarrays produced from devices made in accordance with the presentinvention can be significantly greater than those using prior artstructures. For example, in a SQUID device, a superconducting loop isformed including at least 2 weak links or tunnel barriers. In thepresent invention, it is possible to construct the superconducting loopin such a way that multiple tunnel barriers are formed using the samegrain boundary in order to make all the weak link devices in thesuperconducting loop have essentially identical properties. This cannotbe achieved in prior art structures utilizing several grain boundaries.

SQUID DEVICES

FIGS. 10A and 10B illustrate a SQUID device that was prepared inaccordance with the present invention, while FIG. 11 is a plot ofcritical current I_(c) versus applied magnetic field H, for differentvalues of bias current through the SQUID. This plot was made at 4.6 K.,although the same general characteristics are obtained at temperaturesabove 77 K.

FIG. 10A is a top view of a superconducting layer 24 (on a substrate notshown in this view) having a single grain boundary GB therein separatingthe superconducting grains A and B. The dashed lines 26 and 28 definethe boundaries of the superconducting loop that is to be formed for theSQUID device. Regions of the superconducting layer outside dashed line26 and inside dashed line 28 were removed by laser ablation to leave aloop of superconducting material 30 (FIG. 10B). This superconductingloop is located on the substrate 32, and includes a first portion 30Aand a second portion 30B separated by two grain boundary regions GB.Thus, two tunnel barrier or weak link devices are formed in thesuperconducting loop 30, thereby producing the SQUID.

FIG. 11 shows the response of the SQUID of FIG. 10B to an appliedmagnetic field, and is based on measurements made of the device. Acurrent was produced in the superconducting loop and a magnetic field Hwas applied parallel to the plane of the grain boundaries andperpendicular to the plane of the superconducting layer 24. FIG. 11shows the interferometer response of the SQUID of FIG. 10B to theapplied magnetic flux for three values of bias current I_(b), I_(b2),I_(b3) through the SQUID loop. The precise periodicity and the largemodulation depth as a function of magnetic field clearly demonstratesthe usefulness of these grain boundary junctions in the fabrication ofdevices, such as the SQUID of FIG. 10B.

In addition, the small B_(c) (<1) for these grain boundary junctionseliminates the need for an external resistance shunt to achievenonhysteretic SQUID operation.

A planar spiral coupling coil comprised of all high T_(c)superconducting materials can be made on a separate chip. Afterindividual testing of the coil, the coil and the SQUID of FIG. 10B canbe positioned to achieve optimum coupling of magnetic fields from thecoil to the SQUID. This provides a device with enhanced sensitivity andreproducibility.

What has been described herein are devices based on the first directmeasurements of the critical current of a number of grain boundaries andof their adjoining superconducting grains. The critical current of theboundaries is less than that of the adjoining grains, while thetemperature and magnetic field dependence of the critical currents ofthe grain boundaries indicate that the boundaries are comprised ofregions of weak and strong coupling. The grain boundaries can be used toform structures such as SQUIDS and provide a natural way to develop moreadvanced tunneling structures that can be used for scientific orpractical applications.

The structures of this invention can be used to provide enhanced devicesfor many purposes, including infrared sensors and coherent arrays fordetection and transmission of millimeter waves and linear junctionarrays for voltage standards. For example, a coherent array can befabricated from a number of tunnel devices formed using the same grainboundary as the tunnel barrier or weak link barrier in each device.Since each device can be made very small and have the same propertieswith respect to the superconducting grains and the grain boundary thatis used as the tunnel barrier, coherent arrays having enhancedproperties can be envisioned. Further, since the location andorientation of the grain boundaries can be precisely controlled, designlayouts for a plurality of these devices are easily realized.

While the invention has been described with respect to particularembodiments thereof, it will be appearant to those of skill in the artthat variations may be made therein without departing from the spiritand scope of the present invention. In this regard, the invention isspecifically directed to devices and methods using controlledimperfections (such as grain boundaries) in a superconducting layerwhere the superconducting layer is preferably a high T_(c)superconductor of the type exhibiting a crystalline structure includingcopper oxide planes.

Having thus described our invention what we claim as new and desire tosecure as Letters Patent, is:
 1. A method for making a barrier device ofhigh T_(c) superconducting material, comprising the steps of:providing acrystalline substrate having a grain boundary therein, epitaxiallydepositing a layer of said high T_(c) superconducting material on saidsubstrate to map said grain boundary into said superconducting layerfrom said substrate, patterning said layer of said superconductingmaterial to produce high T_(c) superconducting regions therein on eachside of said grain boundary, and electrically connecting each said highT_(c) superconducting regions to an electrical source for producingcurrent flow between said regions across said grain boundary.
 2. Themethod of claim 1, where said patterning destroys superconductivity inregions of said layer of said high T_(c) superconducting material. 3.The method of claim 1, wherein said superconducting material is a copperoxide material exhibiting superconductivity at temperatures greater than77 K.
 4. The method of claim 1, where said patterning step produces atleast two high T_(c) superconducting regions on each side of said grainboundary.
 5. The method of claim 4, where said at least two high T_(c)superconducting regions are connected to form a loop of said high T_(c)superconducting material having said grain boundary therein.
 6. Themethod of claim 1, where said patterning step produces multiple pairs ofsuperconducting regions straddling said grain boundary.
 7. The method ofclaim 6, where at least two of said pairs of superconducting regions areconnected to form a superconducting loop broken by said grain boundary.